1. Field of the Invention
This application relates to methods of epitaxial deposition of silicon-containing materials. More specifically, the present application relates to methods of cyclical epitaxial deposition and etch.
2. Description of the Related Art
Semiconductor processing is typically used in the fabrication of integrated circuits, which entails particularly stringent quality demands, as well as in a variety of other fields. In forming integrated circuits, epitaxial layers are often desired in selected locations, such as active area mesas among field isolation regions, or even more particularly over defined source and drain regions. While non-epitaxial (amorphous or polycrystalline) material can be selectively removed from over the field isolation regions after a “blanket” deposition, it is typically considered more efficient to simultaneously provide chemical vapor deposition (CVD) and etching chemicals, and to tune conditions to result in zero net deposition over insulative regions and net epitaxial deposition over exposed semiconductor windows. This process, known as “selective” epitaxial deposition, takes advantage of slow nucleation of typical semiconductor deposition processes on insulators like silicon oxide or silicon nitride. Such selective epitaxial deposition also takes advantage of the naturally greater susceptibility of amorphous and polycrystalline materials to etchants, as compared to the susceptibility of epitaxial layers to the same etchants.
The selective formation of epitaxial layers typically involves the use of silicon-containing precursors and etchants along with a carrier gas. One of the most commonly used carrier gases is hydrogen gas (H2). Hydrogen is a useful carrier gas because it can be provided in very high purity and is compatible with silicon deposition. Additionally, it can serve as a reducing agent to form H2O and remove any contaminant oxygen from the substrate or chamber generally. Accordingly, hydrogen remains one of the most widely used carrier gases on its own or in combination with other carrier gases.
One desirable attribute of deposited semiconductor material is the “strain” in the material. The electrical properties of semiconductor materials such as silicon, germanium and silicon germanium alloys are influenced by the degree to which the materials are strained. For example, silicon exhibits enhanced electron mobility under tensile strain, and silicon germanium exhibits enhanced hole mobility under compressive strain. Methods of enhancing the performance of semiconductor materials are of considerable interest and have potential applications in a variety of semiconductor processing applications.
A number of approaches for inducing strain in silicon- and germanium-containing materials have focused on exploiting the differences in the lattice constants between various crystalline materials. For example, the lattice constant for crystalline germanium is 5.65 Å, for crystalline silicon is 5.431 Å, and for diamond carbon is 3.567 Å. Heteroepitaxy involves depositing thin layers of a particular crystalline material onto a different crystalline material in such a way that the deposited layer adopts the lattice constant of the underlying single crystal material. For example, using this approach allows for strained silicon germanium layers to be formed by heteroepitaxial deposition onto single crystal silicon substrates. Since the germanium atoms are slightly larger than the silicon atoms, and the deposited heteroepitaxial silicon germanium is constrained to the smaller lattice constant of the silicon beneath it, the silicon germanium is compressively strained to a degree that varies as a function of the germanium content. Typically, the band gap for the silicon germanium layer decreases monotonically from 1.12 eV for pure silicon to 0.67 eV for pure germanium as the germanium content in heteroepitaxial silicon germanium increases. In another approach, tensile strain can be provided in a thin single crystalline silicon layer by heteroepitaxially depositing the silicon layer onto a relaxed silicon germanium layer. In this example, the heteroepitaxially deposited silicon is strained because its lattice constant is constrained to the larger lattice constant of the relaxed silicon germanium beneath it. The tensile strained heteroepitaxial silicon typically exhibits increased electron mobility. In this approach, the strain is generally developed at the substrate level before the device (for example, a transistor) is fabricated.
In these examples, strain is introduced into single crystalline silicon-containing materials by replacing silicon atoms with other atoms in the lattice structure. This technique is typically referred to as substitutional doping. For example, substitution of germanium atoms for some of the silicon atoms in the lattice structure of single crystalline silicon produces a compressive strain in the resulting substitutionally doped single crystalline silicon material because the germanium atoms are larger than the silicon atoms that they replace. It is possible to introduce a tensile strain into single crystalline silicon by substitutional doping with carbon, because carbon atoms are smaller than the silicon atoms that they replace. Unlike substitutional impurities, non-substitutional impurities will not induce strain.